The present invention is directed to power supply control circuits. More particularly, the invention provides methods and apparatus for a power converter control circuit for controlling the output current-voltage (I-V) characteristics of a switching mode power supply.
Regulated power supplies are indispensable in modern electronics. For example, the power supply in a personal computer often needs to receive power input from various outlets. Desktop and laptop computers often have regulated power supplies on the motherboard to supply power to the CPU, memories, and periphery circuitry. Regulated power supplies are also used in a wide variety of applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) are two alternative control architectures of switching mode power supplies. In recent years, green power supplies are emphasized, which require higher conversion efficiency and lower standby power consumption. In a PWM controlled switching mode power supply, the system can be forced to enter into burst mode in standby conditions to reduce power consumption. In a PFM controlled switching mode power supply, the switching frequency can be reduced in light load conditions. A PFM-controlled switching mode power supply exhibits simple control topology and small quiescent current. Therefore, it is suitable for low cost small output power applications such as battery chargers and adapters, and cooling fan speed control for a personal computer system.
The control operation in a switch mode power supply (SMPS) can be implemented using different techniques, for example, depending on whether the SMPS uses primary-side-regulation (PSR) or secondary-side-regulation (SSR). In SSR, the control of the power supply output characteristics is based on sensing the output current or voltage at the secondary side (output side) of the transformer. The sensed signal is provided by a feedback mechanism, e.g., via an opto-coupler, to the SMPS controller, which controls the primary side current of the transformer.
In contrast, in primary side regulation (PSR), the power supply output current or voltage is not directly sampled at the output side. Instead, a primary side auxiliary winding is used to sense the current or voltage in the primary winding or secondary winding of the transformer. The PSR design is finding increasingly widespread applications, because it can provide high performance and cost effectiveness. In one aspect, the output (secondary side) circuitry of the power supply can be simplified. Moreover, in many power supplies, an auxiliary winding is often used to provide operating power for the controller. In these cases, the auxiliary winding can also provide the sensing function without added cost.
FIG. 1 illustrates an output I-V characteristics for a power supply in applications such as battery chargers and power adapters. As shown, the output voltage versus output current plot exhibits a vertically folded back curve. In a constant voltage (CV) mode 101, the power supply provides a constant output voltage which is substantially independent of the output current. In a constant current (CC) mode 102, the power supply provides a constant current substantially independent of output voltage. In the constant mode, the power supply can provide a constant current at a low output voltage.
Even thought power converters having output current-voltage characteristic of FIG. 1 have been used in many applications, they may not be suited for certain applications, as described in more detail below. Therefore, there is a need for methods and apparatus that can provide more general output current-voltage characteristics in a power supply.